The present invention relates to 4,4'-diphenyl ether-dialdehyde-bis-dimethylacetal and to a process for its preparation.
It is known that anodic alkoxylation of unsubstituted or substituted methylbenzenes of the general formula (I) can lead to the corresponding benzaldehyde-dialkylacetals of the general formula (II):
______________________________________ ##STR3## ##STR4## REACTANT R.sup.1 R.sup.2 PRODUCT ______________________________________ I a H CH.sub.3 II a b CH.sub.3 CH.sub.3 b c -- --OCH.sub.3 CH.sub.3 c C.sub.2 H.sub.5 c' ##STR5## CH.sub.3 d e -- --OCH.sub.2CHCH.sub.2 CH.sub.3 e f ##STR6## CH.sub.3 f g -- --Ot-C.sub.4 H.sub.9 CH.sub.3 g h ##STR7## CH.sub.3 h ______________________________________
According to the article "Anodic Methoxylation of Alkylbenzenes" by K. Sasaki, H. Urata, K. Uneyama and S. Nagaura in Electrochimica Acta, 1967, Vol. 12, pp. 137-146, toluene (Ia) is converted on platinum electrodes in methanol to benzaldehyde-dimethylacetal (IIa) (or, after hydrolysis, to benzaldehyde itself) and to methyl benzyl ether. According to the data in F. Beck, Elektroorganische Chemie, Verlag Chemie-Weinheim, 1974, page 248, the yield of (IIa) is said to be about 10% of the theoretical.
Published French patent application No. 2,351,932 describes the electrochemical oxidation of methylbenzenes, such as toluene (Ia) and xylene (Ib). The following products are obtained in the oxidation of toluene (Ia) in an electrolyte system composed of methanol and a co-solvent, such as methylene chloride, with the use of acidic supporting electrolytes and after subsequent hydrolysis: methyl benzyl ether, benzaldehyde and p-methoxybenzaldehyde, and in addition also, inter alia, o-methoxy-benzaldehyde or p-methoxytoluene. The yields of benzaldehyde are 3.6 to 13.2% of the theoretical. In the corresponding anodic oxidation of p-xylene (Ib), the reaction products (no yield data are given) are methyl-p-xylyl ether, 4-methyl-benzaldehyde, methyl-4-methyl-benzoate and 2-methoxy-4-methyl-benzaldehyde.
According to the article "Anodic Substitution and Addition Reactions" by S. Tsutsumi and K. Koyama in Discussions of the Faraday Society, 1968, No. 45, pp. 247-253, ring-substituted methoxy derivatives of toluene or of tolunitrile are also obtained in the anodic cyanation of toluene on platinum electrodes in a methanol/NaCN system.
More recently, remarkable increases in the selectivity of the electrochemical alkoxylation of toluenes substituted in the 4-position have also been disclosed.
In the article "Nuclear Cyanation of Methylanisoles" by K. Yoshida, M. Shigi and T. Fueno in J. Org. Chem., 1975, Vol. 40, No. 1, pp. 63-66, the reaction of 4-methoxytoluene (Ic) in a methanolic solution of NaCN or Na acetate is described. In this reaction, 4-(methoxymethyl)-anisole and, in a material (current) yield of 15% (24%) of the theoretical, anisaldehyde-dimethylacetal (IIc) are also formed, in addition to the respective ring-substitution products.
A. Nilsson, U. Palmquist, T. Pettersson and A. Ronlan, "Methoxylation of Methyl-substituted Benzene and Anisole Derivatives, and the Synthesis of Aromatic Aldehydes by Anodic Oxidation" in J. Chem. Soc., Perkin Transactions I, 1978, pp. 708-715, have succeeded in methoxylating p-xylene (Ib) and 4-methoxytoluene (Ic) in methanol with the use of NaOCH.sub.3 /LiBF.sub.4 or NaOCH.sub.3 supporting electrolytes on a carbon anode at about 10.degree. C. to give compound (IIb) in a material (current) yield of 57% of the theoretical and compound (IIc) in a material (current) yield of 66% of the theoretical.
In Published European application No. 0,011,712 (see also German Offenlegungsschrift No. 2,848,397), benzaldehyde-dialkylacetals, substituted in the 4-position, of the general formula ##STR8## are described, wherein R.sup.1, inter alia, can also represent a phenyl radical or benzyl radical and R.sup.2 can represent an alkyl radical having 1 to 4 C atoms. These compounds are prepared by electrochemically oxidizing methylbenzenes (toluenes), substituted in the 4-position, of the general formula ##STR9## in the presence of an alcohol R.sup.2 --O--H (R.sup.1 and R.sup.2 having the meaning indicated above) and of a supporting salt. Examples of suitable supporting salts are: fluorides such as KF, tetrafluoborates such as Et.sub.4 NBF.sub.4 (where Et=ethyl), perchlorates such as Et.sub.4 NClO.sub.4, sulfates such as Et.sub.4 NSO.sub.4 Et, alcoholates such as NaOCH.sub.3 and hydroxides such as KOH. The quantitative proportions of the components should be between about 5 and 50% by weight of the substituted methylbenzenes, between about 50 and 95% by weight of the alcohol and between about 0.5 and 15% by weight of the supporting salt. Graphite, graphite-filled plastics and noble metals are mentioned as the anode materials, and graphite, iron, steel, lead and noble metals are mentioned as the cathode materials. The current densities are from 1 to 20 A/dm.sup.2, and the electrolysis temperature is between about 0.degree. and 60.degree. C. In detail, the anodic methoxylation or ethoxylation, respectively, of p-xylene (Ib) gives a material yield (current yield) of 32% (18%) of the theoretical of 4-methyl-benzaldehyde-dimethylacetal (IIb), that of 4-methoxytoluene (Ic) gives 42.4% (22%) to 73.1% (56.5%) of anisaldehyde-dimethylacetal (IIc) or 53.4% (-) of anisaldehyde-diethylacetal (IIc'), that of 4-benzyloxytoluene (Id) gives 62.1% (47.9%) of 4-benzyloxy-benzaldehyde-dimethylacetal (IId), that of 4-allyloxytoluene (Ie) gives 36.3% (10.8%) of 4-allyloxy-benzaldehyde-dimethylacetal (IIe), that of 4-phenoxy toluene (If) gives 39.2% (14.3%) of 4-phenoxy-benzaldehyde-dimethylacetal (IIf), that of 4-t-butoxytoluene (Ig) gives 52.5% (19.2%) of 4-t-butoxy-benzaldehyde-dimethylacetal (IIg) and that of 4-N,N-dimethylamino-carboxyl-toluene gives 40.4% (-) of 4-(N,N-dimethylamino-carboxyl)-benzaldehyde-dimethylacetal.
In the process for the preparation of substituted benzaldehyde-dialkylacetals according to Published European application No. 0,012,240, inter alia, methylbenzenes (toluenes), substituted in the 4-position, of the general formula ##STR10## in which R.sup.1 can, inter alia, represent an aryloxy or aralkoxy radical, are electrochemically oxidized in solution in an alcohol of the formula R.sup.2 --O--H (R.sup.2 =alkyl) and in the presence of a supporting salt at a current density of 0.1 to 50 A/dm.sup.2. The aryloxy and aralkoxy radicals mentioned are, inter alia, phenoxy, naphthyloxy, anthryloxy, benzyloxy and 2-phenyl-ethoxy, according to which p-phenoxytoluene (=1-methyl-4-phenoxy-benzene) should then, inter alia, also be included with the methylbenzenes substituted in the 4-position. Suitable supporting salts include tetraethylammonium-p-toluene-sulfonate, tetraethylammonium-ethyl-sulfate or tetramethylammonium dimethyl phosphate. Graphite, lead dioxide and noble metals are mentioned as anode materials, and copper, nickel, steel, platinum and graphite are mentioned as cathode materials. In detail, the anodic methoxylation of p-xylene (Ib) gives a material yield (current yield) of 64% (-) of 4-methylbenzaldehyde-dimethylacetal (IIb), that of 4-methoxytoluene (Ic) gives 67% (71%) to 95% (-) of 4-methoxybenzaldehyde-dimethylacetal and that of 4-t-butoxytoluene (Ig) gives 55% (-) to 92% (-) of 4-t-butoxybenzaldehyde-dimethylacetal (IIg).
None of these numerous publications, however, contains even the slightest hint to the effect that it would be possible to alkoxylate aromatic compounds which have 2 methyl groups located on an aromatic system in the molecule, to simultaneously alkoxylate both methyl groups to give the corresponding aromatic dialdehyde-bis-dialkylacetals. Thus, for example, as described by A. Nilsson et al. (cited above), the anodic oxidation of p-xylene (Ib), depending on the experimental conditions (page 709, bottom of right-hand column) either leads via a nuclear methoxylation to 1-methyl-1-methoxy-4-methyl-4-methoxy-cyclohexa-2,5-diene (III) or leads via a side-chain methoxylation to (4-methyl-benzyl)-methyl-ether (IV), to 4-methylbenzaldehyde-dimethylacetal (IIb) or, in the last oxidation stage, to the ortho-ester of p-toluic acid (V): ##STR11##
The even more recent publications (German Offenlegungsschrift No. 2,848,397, European application No. 0,011,712 or European application No. 0,012,240) likewise do not contain any hint to the effect that, in the anodic methoxylation of, e.g., p-xylene (Ib), even traces of terephthaldialdehyde-bis-dimethylacetal (VI) ##STR12## could be formed. Even with the most sensitive analytical methods, such as a combination of gas chromatography and mass spectroscopy according to Published French application No. 2,351,932, it was not possible to detect either the dialdehyde or the corresponding bis-dimethylacetal in this reaction.
These findings coincide with the results in an article "Anodic Oxidation of Arylcyclopropanes" by T. Shono and Y. Matsumura in J. Org. Chem., 1970, Vol. 35, No. 12, pp. 4157-4160, according to which either an .alpha.-methoxylation to give 2-(p-tolyl)-2-methoxypropane, or the secondary product p-isopropenyl-toluene obtained therefrom, or an .alpha.'-methoxylation to give (p-i-propyl-benzyl)-methyl-ether takes place in the anodic methoxylation of 4-i-propyltoluene. Accordingly, once a methoxy group has entered into the .alpha.-position, a possible subsequent .alpha.'-substitution in the same molecule is completely inhibited.